1. Field of the Invention
The present invention relates to the field of optoelectronics.
In particular, the aim of the invention is the production of integrated laser-guide-modulator sources.
There may be many applications of the present invention, such as:
high-rate optical transmission of the NRZ (tunable laser-modulator) type or RZ (OTDM system) type;
distribution;
radio-on-fiber (production of BLU circuit).
Many methods have already been explored in order to achieve active-passive coupling (for example for laser-modulator integration) on the same substrate.
Mention may especially be made of four methods described in the literature: 1) end-to-end coupling, 2) organometallic-selective epitaxy, 3) evanescent coupling and 4) xe2x80x9csingle structurexe2x80x9d.
End-to-end coupling consists in firstly growing epitaxially a laser structure, in etching this first structure at the locations dedicated to a modulator and then in carrying out, at these locations, the epitaxy of the structure of the modulator. This technique involves at least three epitaxy steps [1]. An example of a structure obtained with this method is illustrated in FIG. 1, in which the wavelengths xcex1, xcex2, xcex3 symbolically indicate the various transition energies of each section.
Organometallic-selective epitaxy requires only a single epitaxy step. Its principle is based on the possibility of being able to modify locally, by the presence of a dielectric mask of well-defined geometry, the thickness and the composition of the material, and consequently the wavelength corresponding to the band gap width of the active region of the epitaxially grown structure (for example to that of the structure on an unmasked region) [2]. It is thus possible to obtain, side by side, several regions having different band gaps corresponding to as many optical functions. For DBR laser-modulator integration for example (DBR standing for Distributed Bragg Reflector, that is to say a laser with a distributed Bragg grating), these three gaps correspond to the active section of the laser, to a Bragg region and to a modulator. FIG. 2 illustrates a cross-sectional schematic view of growth by this method of selective epitaxy. In this FIG. 2, the transition energy of each section is represented by its equivalent wavelength.
Evanescent (or vertical) coupling allows the formation of two guiding structures placed one on top of the other in a single epitaxy step. An etching operation makes it possible to reveal the regions where the upper layer is not desired, thus revealing the modulator or guide section. The optical mode remains confined within the upper layer of higher refractive index and then, from the laser-modulator interface, is propagated into the lower layer [3], [4]. FIG. 3 shows the basic principle of the method of evanescent coupling. The optical mode passes from one layer to another via the set of index and geometry differences of the guides used.
The method known as the xe2x80x9csingle structurexe2x80x9d method consists in using the same multiple quantum well stack for each of the sections of the component (3 in the case of DBR laser-modulator integration)[5],[6]. This method relies on the optical gain spectrum broadening effects for strained semiconductors when carriers are injected into them. This property makes it possible to obtain an optical amplification effect and therefore laser emission for a lower energy than the transition energy (of the semiconductor without injected carriers). The wavelength compatibility between the laser and the modulator (typically, 1.55 xcexcm in the case of the laser and 1.50 xcexcm in the case of the modulator) is obtained by a slight red shift of the Bragg wavelength. FIG. 4 shows the basic principle of the single structure. The difference in transition energy between the two sections is obtained by narrowing the band gap range carrier injection.
All these known techniques each seem to have intrinsic advantages. However, none of them is truly satisfactory.
With regard to end-to-end coupling, the main advantage is the optimization of the two structures separately. However, the method does involve a good control of the etching and of the epitaxial regrowth on a substrate not prepared for epitaxy, so as to align the layers of the various sections one with respect to another. In addition, the coupling between the two sections is responsible for losses and reflections which may disturb the operation of integrated sources at high-frequency. Moreover, the process requires several epitaxy steps, which incurs a cost.
Localized epitaxy makes it possible to obtain excellent results in terms of component integration. The coupling losses between the various sections are in this case very small and the band gaps of the various sections can be adjusted to precise values according to the mask (for example based on silicon nitride) used. However, its main drawback stems from the fact that the same type of structure is used for each section of the component. Unlike end-to-end coupling, the structures of each section of the component cannot therefore be adjusted separately. For example, for DBR laser-modulator integration, the maximum tunability which can be obtained is limited by the fact that the thickness of the Bragg section is necessarily smaller than that of the active region of the laser. Moreover, a critical surface preparation step is also needed.
With regard to evanescent coupling, the main advantage is the almost independent optimization of two sections of the component (lower structure and upper structure). Moreover, this is a relatively simple technique to employ. On the down side, it is difficult to integrate more than two different sections. This is a handicap in the case, for example, of DBR-modulator integration in which three sections are needed.
The main advantage of the xe2x80x9csingle structurexe2x80x9d is its simplicity. However, as in the case of selective epitaxy, the various sections cannot be optimized separately. This results in a compromise which allows integration only of optical functions exhibiting a relatively small wavelength shift (laser-modulator, modulator-amplifier, etc.). Integration of a laser with a passive guide is impossible as it requires too great a wavelength shift.
Document U.S. Pat. No. 4,054,363 describes a guide formed from a heterostructure of the optical integrated circuit type, provided with a thin film element such as a semiconductor laser coupled through a directional coupler to a guide having a low transmission loss.
The document by Z. M. Chuang et al. xe2x80x9cPhotonic integrated tunable receivers with optical preamplifiers for direct detectionxe2x80x9d, Applied physics Letters, US, American Institute of Physics, New York, Vol. 63, No. 7, pages 880-882 describes an integrated tunable photonic receiver which comprises, in series, an optical preamplifier, a codirectional coupler forming a filter, and a photodetector.
The object of the present invention is now to provide novel means making it possible to integrate, using a simple technique compatible with industrial processes, a minimum of three sections of different band gap energies with, if possible, structures which are optimized for each section.
Integration by evanescent coupling allows only two optimized sections to be integrated. To integrate three different sections with optimized structures for each is theoretically possible with end-to-end coupling, but three epitaxy stages are needed in this case, with the drawbacks that were mentioned above. Organometallic-selective epitaxy makes it possible to integrate three different sections, but in this case the structures cannot be optimized independently. The same reasoning is valid for the single structure. Admittedly, in this case, very good results have been shown for DFB laser-modulator integration (with 2 sections) (DFB stands for Distributed Feedback, that is to say a laser equipped with a periodic Bragg grating). However, matters become complicated if it is desired to integrate the modulator with a tunable (DBR) laser over a tunability range of 10 nm for example. In this case, a third specific section (Bragg section) must be introduced between the active region of the laser and the modulator.
The aforementioned objective is achieved according to the present invention by means of an optoelectronic system comprising at least three sections corresponding to specific respective functions and having different respective band gap energies, at least for the adjacent pairs of sections, consisting of at least two layers superposed by epitaxy, the upper layer being etched in order to define said sections in the form of two separate end sections limited in the upper layer on each side of an intermediate section defined in the lower layer, and to allow coupling between the intermediate section and each of the end sections which flank it, by evanescent coupling, in which system the length xe2x80x9cLxe2x80x9d of the intermediate section, in which a Bragg grating is etched, is such that the product Kxc3x97L (where K is the coupling coefficient of the grating) is of the order of 1.
For example, for K of about 50 cmxe2x88x921, this length L is bout 200 xcexcm.
The present invention also relates to a process for producing the aforementioned system.
The aforementioned concept according to the present invention makes it possible to produce, using a simple and industrializable technology, lasers that can be tuned over more than 10 nm, soliton sources, tandems of modulators, integrated sources for BLU, etc.